Silicon dioxide has become the preferred hard-mask patterning material for plasma etching deep trenches in silicon. In a chemically-halogenated plasma (the standard environment for such etches), silicon dioxide has a reactivity (also termed "selectivity") approximately 1/40 to 1/10 that of silicon. In other words, the silicon dioxide mask is consumed during the etch at a rate of 1/40 to 1/10 the rate at which silicon is consumed. For a given mask thickness, mask selectivity effectively limits trench depth. At the high end of the selectivity range, it is necessary to alter the chemistry of the plasma with chemical species that tend to cause deposition of unwanted materials on the surfaces of the etch chamber. For example, the presence of oxygen radicals and ions, chemical species that enhance selectivity of silicon over oxide during a halogenated-plasma etch, react with silicon to form silicon dioxide that is deposited as a glass layer on etch chamber walls. Deposited materials represent a potential source of pollution that must be periodically removed from the chamber. In addition, since mask selectivity may also be etch-rate dependent, it may be necessary to perform a plasma etch at an inconveniently slow rate in order to achieve a desired trench depth with a mask of optimal maximum thickness. Furthermore, as silicon dioxide masking material is eroded during a plasma etch, it may be redeposited on trench sidewalls near the mouth of the trench, thus, further complicating the fabrication process. Finally, if MOS gates have been created prior to an anisotropic plasma trench etch, removal of a silicon dioxide patterning mask with an isotropic oxide etch subsequent to the trench etch may compromise the integrity of existing gate oxide. This is especially true at gate edges.
What is needed is a patterning mask material that is essentially impervious to halogenated-plasma etches, and that does not require exotic modification of a halogen plasma. Ideally, the mask could be removed with a chemical wet etch that would not etch existing circuitry components.